Presently, fuel cell systems are being proposed for use as a power source in a wide variety of commercial and non-commercial applications. In particular, fuel cell systems are increasingly being used as a replacement for internal combustion engines in motor vehicles. Such a system is disclosed in commonly owned U.S. Pat. No. 7,459,227, hereby incorporated herein by reference in its entirety. Typically, the fuel cell system generates electricity which is then used to charge batteries or to provide power to an electric motor. Fuel cell systems may also be used as stationary electric power plants in buildings and residences, and as portable power in video cameras, computers, and the like.
The fuel cell system is typically comprised of a plurality of fuel cells bundled together and arranged in electrical series to form a stack. Since the fuel cells can be assembled into stacks of varying sizes, the fuel cell system can be designed to produce a desired energy output level providing flexibility of design for different applications. The fuel cells are electrochemical devices which directly combine gaseous reactants such as a fuel (e.g. hydrogen) and an oxidant (e.g. oxygen) to produce electricity. The oxygen is typically supplied by an air stream. The gaseous reactants combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example. The basic process employed by the fuel cell system is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products.
Different fuel cell types can be provided such as phosphoric acid, alkaline, molten carbonate, solid oxide, and proton exchange membrane (PEM), for example. The basic components of a PEM-type fuel cell are two electrodes separated by a polymer electrolyte membrane. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between anode and cathode diffusion media (hereinafter “DMs”) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The DMs serve as the primary current collectors for the anode and cathode, as well as provide mechanical support for the MEA. Alternatively, the DMs may contain the catalyst layer and be in contact with the membrane. The DMs and MEA are pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct electrical current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack in the case of monopolar plates at the end of the stack.
The secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the primary current collector and define a plurality of grooves or flow channels therebetween. The channels supply the fuel and the oxidant to the electrodes on either side of the PEM. In particular, the fuel flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxidant flows through the channels to the cathode where the oxidant attracts the protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxidant to produce water vapor at the cathode side.
In order to perform within a desired efficiency range, it is desirable to maintain the membranes in a humidified condition. Therefore, it is necessary to provide a means for maintaining the fuel cell membranes in the humidified condition. The humidified condition helps avoid a shortened life of the membranes, as well as to maintain the desired efficiency of operation. For example, lower water content of the membrane leads to a higher proton conduction resistance, thus resulting in a higher ohmic voltage loss. The humidification of the feed gases, in particular at the cathode inlet, is desirable in order to maintain sufficient water content in the membrane. Humidification in a fuel cell is discussed in commonly owned U.S. Pat. No. 7,036,466; commonly owned U.S. Pat. App. Pub. No. 2006/0029837; and commonly owned U.S. Pat. No. 7,572,531, each of which is hereby incorporated herein by reference in its entirety.
To maintain a desired moisture level, an air humidifier is frequently used to humidify a supply stream of the air used in the fuel cell. The air humidifier normally consists of a round or box type air humidification module that is installed into a housing of the air humidifier. Examples of this type of air humidifier are shown and described in U.S. Pat. No. 7,156,379, hereby incorporated herein by reference in its entirety, and U.S. Pat. No. 6,471,195, hereby incorporated herein by reference in its entirety.
Membrane humidifiers, such as water vapor transfer (WVT) units, have also been utilized to fulfill fuel cell humidification requirements. For the automotive fuel cell humidification application, such a membrane humidifier needs to be compact, exhibit low pressure drop, and have high performance characteristics. Typical WVT units include a wet plate that includes a plurality of flow channels formed therein adjacent a diffusion media. The flow channels of the wet plate are adapted to convey a wet fluid from the cathode of the fuel cell to the exhaust. Typical WVT units also include a dry plate that includes a plurality of flow channels formed therein adjacent a diffusion media. The flow channels of the dry plate are adapted to convey a dry fluid from a source of gas to the cathode of the fuel cell. A similar WVT unit can be used for an anode side of the fuel cell, or otherwise as desired.
One type of WVT unit is a cross-flow WVT unit in which a direction of flow of the wet fluid in the wet plate is perpendicular to a direction of flow of the dry fluid in the dry plate. Accordingly, the flow channels of the wet plate are formed perpendicular to the flow channels of the dry plate. The flow channels of the wet and dry plates are typically uniformly spaced apart at a desired interval. The uniform spacing of the flow channels of the plates results in uneven relative humidity distribution across outlets of the dry plate. Relative humidity levels are typically highest in the outlets of the flow channels of the dry plate adjacent inlets of the flow channels of the wet plate. Conversely, the relative humidity levels are typically lowest in the outlets of the flow channels of the dry plate adjacent outlets of the flow channels of the wet plate.
Particularly at idle conditions of the fuel cell system, when water vapor transfer efficiency is maximized and an operating temperature of the fuel cell system is minimized, the relative humidity in the flow channels of the dry plates can reach 100%. This can cause liquid water to form at the outlets of the flow channels of the dry plate adjacent the inlets of the flow channels of the wet plate and enter the fuel cell stack. Liquid water in the fuel cell stack decreases durability and can result in unstable performance of the fuel cell system. Typically, a liquid water separator is employed to militate against the liquid water from entering the fuel cell stack, adding cost and complexity to the fuel cell system. However, an effectiveness of the liquid water separator may be less than optimal, permitting a portion of the liquid water to enter the fuel cell stack.
It would be desirable to produce a fuel cell system including a fluid flow distribution feature, which minimizes a variation of relative humidity across the outlets of the dry flow channels of the WVT unit, as well as a cost and a complexity of the fuel cell system.